In the aluminum industry, dispersoid-forming elements such as Zr, Mn, Cr, V, Ti, Sc and Hf are used to increase recrystallization temperature and to control the grain structure in cast and wrought products. Many different methods have been employed to add these types of alloying elements to molten metals. Typically, master alloys which contain the desired elements are added directly to the melt in the forms of a cast lump, bar, waffle or added as briquettes composed of mixtures of aluminum and elemental powders or chips.
The alloying elements in the master alloys are normally present in a form of coarse intermetallics such as for example Al.sub.3 Zr. These intermetallics require superheat and a long period of holding time to be dissolved in the melt. The heavy intermetallics also tend to settle to the bottom of the holding furnace due to gravity. Thus, master alloys are generally added in the melting or holding furnace to allow sufficient time for the intermetallics to dissolve in the superheated melt which is occasionally stirred.
In addition, the level of these desirable dispersoid-forming elements in the commercial aluminum alloys has been limited to the liquid solubility at peritectic reaction temperature. For example, in the case of aluminum binary systems, the maximum liquid solubility of Zr, Cr, V and Hf is 0.12, 0.37, 0.2 and 0.2 wt. %, respectively. In commercial aluminum alloys, these maximum limits of liquid solubility at peritectic temperatures will be reduced even further. Casting of aluminum alloys containing dispersoid elements at levels above their natural saturation limit can result in formation of undesirable coarse primary intermetallics in the molten metal.
If coarse intermetallics are not filtered out of the molten metal, they will adversely affect the ability to cast the metal as well as the mechanical properties of the end product by reducing ductility, fracture toughness, or fatigue properties. Since coarse primary intermetallics can rapidly nucleate and grow in melts which exceed the maximum solubility limit, the conventional alloying approach is to add dispersoid-forming elements in the melting or holding furnace in amounts below the liquid saturation limit.
It would be highly desirable to form metal which has been cast such that it contains dispersoid-forming elements at a level greater than the liquid solubility limit of the elements. Supersaturated levels of dispersoid-forming elements in solid solution will increase the number of nucleation sites which form fine dispersoids during preheating of the cast alloy, which enables the recrystallization temperature to be increased, and inhibits grain growth during hot working.
For structural applications, a fine grain unrecrystallized microstructure has a better combination of strength, elongation and toughness than a coarse grain recrystallized alloy. Metallurgically, a high volume fraction of fine dispersoids which are less than about 0.1 microns in size are useful for retaining a fine grain unrecrystallized microstructure.
Currently the volume fraction of dispersoids which can be formed is limited by the liquid solubility of the dispersoid-forming elements in the alloy.
It is against this background that the present invention was made.
Accordingly, it is a principal object of this invention to provide aluminum alloys having high levels of fine dispersoids.
It is a further object of the present invention to provide a method for increasing the amount of dispersoid-forming elements in solid solution which is not limited to the liquid solubility level.
Another object of the invention is to provide a method to increase the volume fraction of dispersoids formed by precipitating from a supersaturated solid solution.
Yet another object of the present invention is to provide a method for casting aluminum alloys with supersaturated levels of dispersoid-forming elements.
Yet it is another object of this invention to provide aluminum alloys having levels of Zr greater than about 0.12 wt. %.
Yet it is another object of this invention to provide aluminum alloys having levels of Mn greater than about 2.06 wt. %.
Yet it is another object of this invention to provide aluminum alloys having levels of Cr greater than about 0.37 wt. %.
Yet it is another object of this invention to provide aluminum alloys having levels of V greater than about 0.2 wt. %.
Yet it is another object of this invention to provide aluminum alloys having levels of Ti greater than about 0.14 wt. %.
Yet it is another object of this invention to provide aluminum alloys having levels of Hf greater than about 0.20 wt. %.
Yet it is another object of this invention to provide aluminum alloys having levels of Y greater than about 0.16 wt. %.
Yet it is another object of this invention to provide aluminum alloys having levels of Nb greater than about 0.016 wt. %.
Yet it is another object of this invention to provide aluminum alloys having levels of Sc greater than about 0.47 wt. %
It is a further object of this invention to provide a method for casting aluminum alloys having levels of dispersoid-forming elements in solid solution greater than the liquid solubility limits.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.